• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Need a little help with a power supply I designed.

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bill mountain

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Joined 2010
Michael Bean said:
I generally try to keep supply voltages to no more than about 85% of maximum for an adequate safety margin, so with a 300v plate limit you probably should be around 255 volts. In theory, with 120 volts ac into a voltage doubler, you'll have about 330 volts dc. You can use a resistive voltage divider to reduce plate voltage to 300 vdc at power up, which would then drop to operating voltage as the tube(s) come into operation. You can start with the equivalent operating resistance of the tube by dividing the nominal operating voltage by the operating current. Then calculate voltage divider from there using Ohms'Law.
Give it a try and see what you come up with.

Mike
Click to expand...


So...would I put a resistor to ground after the plate resistor? Or a separate voltage divider before the plate?
 
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bill mountain

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I don't have the means to redraw my schematic right now but I'm thinking that I could put a voltage divider where the 22k resistor is before the last smoothing cap. I'll try that this evening and report back if anyone is interested

I think 100K should do it.
 
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Michael Bean

Michael Bean

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Joined 2004
Like this;

voltage divider.JPG

Caldulate divider values with R(parallel) in parallel with plate resistance.

Mike
 
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stratus46

Member
Joined 2009
bill mountain said:
So what you're saying is that my voltage is even higher? I understand RMS and Peak in relation to power amp output. Is it the same for DC voltage?
Click to expand...

What he's saying is your 120Volt AC is 339.4 Volts peak to peak. Your capacitors charge to the peak value. The transformer is likely a little high as it's rated for 120 Volts at the rated current so at low current will be even higher. There's your 370 Volts.

G²
 
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bill mountain

Member
Joined 2010
Michael Bean said:
Like this;

View attachment 232684

Caldulate divider values with R(parallel) in parallel with plate resistance.

Mike
Click to expand...

I said it takes a while for me to understand stuff! I could be wrong but now I'm thinking that I need to calculate my plate resistance then add that in parallel with the the R(parallel) then choose my R(series) off of that. Is this correct?
 
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Michael Bean

Michael Bean

Member
Joined 2004
Yes, and when when you're done double check all voltage drops and currents to be sure you get the expected results. Once that's done, put it together and test it, I always use a "light bulb tester" when powering up anything the first time. And be sure to be careful working with high voltages, death really sucks.

Mike
 
Michael Bean

Michael Bean

Member
Joined 2004
I obviuosly don't think you're wrong to agree with me.:) When I first got started messing around with electronics, I got off on the wrong foot, it took me a long time to get my head right about the fundamentals. Once you have a good, solid understanding of the basics, the more advanced stuff just kind of falls into place.

Mike
 
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bill mountain

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Joined 2010
OK. So this is what I have figured out. The primary tube I'm going to use will be a 12AX7 which has an anode resistance of 80k. I will use an 82k for the parallel resistor in my voltage divider to give me a total parallel resistance of about 40.5k. If I use a series resistor of 10k then the Vout will be about 297V.

I'll plug this in tonight and let you know how it works out.

Would there be any benefit if I were to place any smoothing caps after the voltage divider? Would that effect the calculation of the 2 parallel resistors?

Thanks!

EDIT: OK so I decided to go a little lower with my B+ to be on the safe side. I will use a 22k to get a parallel resistance of 17.25k. I will use a series resistor of 5.6k which will give me about 280V to work with. I feel much better about these numbers.
 
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bill mountain

Member
Joined 2010
bill mountain said:
OK. So this is what I have figured out. The primary tube I'm going to use will be a 12AX7 which has an anode resistance of 80k. I will use an 82k for the parallel resistor in my voltage divider to give me a total parallel resistance of about 40.5k. If I use a series resistor of 10k then the Vout will be about 297V.

I'll plug this in tonight and let you know how it works out.

Would there be any benefit if I were to place any smoothing caps after the voltage divider? Would that effect the calculation of the 2 parallel resistors?

Thanks!

EDIT: OK so I decided to go a little lower with my B+ to be on the safe side. I will use a 22k to get a parallel resistance of 17.25k. I will use a series resistor of 5.6k which will give me about 280V to work with. I feel much better about these numbers.
Click to expand...

EDIT #2: The more I think about it the more I like my second set of resistor values because no matter what tube I use, the parallel resistance can't be more than 22k which when used with the 5.6k series resistor yields about 295 volts.
 
Michael Bean

Michael Bean

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Joined 2004
I just took a look at 12ax7 data sheet, http://www.drtube.com/datasheets/12ax7-sylvania1955.pdf and the plate resistance is 80k at 100volts, and 62.5k at 250volts. And it appears you will be running the plate current too high with a 10k series resistor, so you'll need to re-think that. Cap after the divider won't affect voltage. And don't forget the power dissapation in the resistors, and be sure the resistors you use have a high enough voltage rating too.

Mike
 
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bill mountain

Member
Joined 2010
Michael Bean said:
I just took a look at 12ax7 data sheet, http://www.drtube.com/datasheets/12ax7-sylvania1955.pdf and the plate resistance is 80k at 100volts, and 62.5k at 250volts. And it appears you will be running the plate current too high with a 10k series resistor, so you'll need to re-think that. Cap after the divider won't affect voltage. And don't forget the power dissapation in the resistors, and be sure the resistors you use have a high enough voltage rating too.

Mike
Click to expand...

I'm not putting a 10k on the plate. My plate resistor is going to be 100k or 220k. I wasn't factoring that into my equation. I'm going to do the voltage divider with a 5.6k and 22k (see my edits) then one more smoothing cap for the power supply. Then the B+ from the power supply (about 290V) will be hooked to the plate resistor. I did my math thinking that the actual plate resistance (80k or 62.5k) was going to be in parallel with the 22k in my voltage divider. I didn't figure the plate resistor since the plate resistor won't be in effect at start up. I wired it up with my multimeter attached to the B+ output. The B+ sat around 270 volts with the tube in place.

I also put a 560 ohm resistor in series with each of the heater windings and the voltage sat around 12.4VAC with the tube in place so, I consider my heater voltage problem solved.

I don't have voltage ratings for my resistors but are all 2W and they seem to be holding up.

I'm sorry if I have confused you. I can draw and post an updated schematic tomorrow.

Thanks so much for all of your help and everyone else in this thread who gave me a hand!
 
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